1. Technical Field
The disclosure relates in general to a method and an apparatus for motion estimation, and more particularly to a method using fractional motion estimation and an apparatus for motion estimation.
2. Description of the Related Art
The H.264/Advanced Video Coding (AVC) video compression standard stipulated by the International Telecommunication Union (ITU-T) and the International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC) achieves high compression ratio and has been widely used in many applications such as blue-ray disc, Internet Protocol Television (IPTV), and High Definition Television (HDTV) broadcasting. The compression standard employs two prediction methods for predicting the pixels of the currently encoded macroblock. One method is intra frame prediction, which predicts the pixels of the currently encoded macroblock according to the spatial correlation between adjacent macroblocks. The other method is inter frame prediction, which predicts the pixels of the currently encoded macroblock according to the time correlation between consecutive frames. The method for motion estimation finds the best matching region from the previous frame for each of the macroblocks that need to be encoded, and generates a corresponding motion vector. In addition to performing integer motion estimation with respect to the integer pixels, the method for motion estimation could perform fractional motion estimation to increase the accuracy level up to ½ or ¼ pixel.
The inter frame prediction, despite having great contribution in terms of compression performance, requires a large amount of computation time. For example, the H.264/AVC standard provides motion estimation for variable-size block modes, but motion estimation occupies about 60%-90% of the encoding time. Moreover, in the H.264 reference software, to obtain the best block mode, full search is performed with respect to all block modes, wherein integer motion estimation and fractional motion estimation are respectively performed. Thus, the complexity of the encoding computation according to H.264 is much higher than the previous video standard such as MPEG-1 or MPEG-2, which adopts fix-sized macroblock for motion estimation. The application of real-time encoding, such as the application of video recording in a hand-held apparatus, will use a large amount of computation resources or require a high-performance processor for encoding, thus increasing the cost for the implementation of an encoder.